In recent years, digital cameras having a multiple exposure function for synthesizing a plurality of color-processed color image data to generate an image in which a plurality of scenes appear to be present on a single image have come to be known. JP2004-48663A discloses a technique for realizing image synthesis processing excluding the effects of gamma conversion and so on. In this technique, synthesis is performed after converting color-processed color image data into pre-color processing color image data, whereupon the color processing is implemented anew.